Most populations of the woolly mammoth in North America and Eurasia, as well all the Columbian mammoths in North America, died out around the time of the last glacial retreat, as part of a mass extinction of megafauna in northern Eurasia and the Americas. Until recently, the last woolly mammoths were generally assumed to have vanished from Europe and southern Siberia about 12,000 years ago, but new findings show some were still present there about 10,000 years ago. In a paper published June 12 in the journal Nature Communications, UCLA researchers and colleagues reveal that not long after the last ice age, the last woolly mammoths succumbed to a lethal combination of climate warming, encroaching humans and habitat change — the same threats facing many species today.

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"We were interested to know what happened to this species during the climate warming at the end of the last ice age because we were looking for insights into what might happen today due to human-induced climate change," said Glen MacDonald, director of UCLA's Institute of the Environment and Sustainability (IoES). "The answer to why woolly mammoths died off sounds a lot like what we expect with future climate warming."

Their work shows that although hunting by people may have contributed to the demise of woolly mammoths, contact with humans isn't the only reason this furry branch of the Elephantidae family went extinct. By creating the most complete maps to date of all the changes happening thousands of years ago, the researchers showed that the extinction didn't line up with any single change but with the combination of several new environmental pressures on woolly mammoths.

When the last ice age ended about 15,000 years ago, woolly mammoths were on the rise. Warming melted glaciers, but the still-chilly temperatures were perfect for such furry animals and kept plant life in just the right balance. It was good weather for growing mammoths' preferred foods, while still too cold for the development of thick forests to block their paths or for marshy peatlands to slow their stride.

But the research explains that the end was coming for the last of the woolly mammoths, who inhabited Beringia, a chilly region linked by the Bering Strait that included wide swaths of Alaska, the Yukon and Siberia.

Though humans had hunted woolly mammoths in Siberia for millennia, it wasn't until the last ice age that people crossed the Bering Strait and began hunting them in Alaska and the Yukon for the first time. After a harsh, 1,500-year cold snap called the Younger Dryas about 13,000 years ago, the climate began to get even warmer. The rising temperatures led to a decline in woolly mammoths' favored foods, like grasses and willows, and encouraged the growth of conifers and potentially toxic birch. Marshy peatlands developed, forcing the mammoths to struggle through difficult and nutritionally poor terrain, and forests became more abundant, squeezing mammoths out of their former territory.

"It's not just the climate change that killed them off," MacDonald said. "It's the habitat change and human pressure. Hunting expanded at the same time that the habitat became less amenable."

Most of the woolly mammoths died about 10,000 years ago, with the final small populations, which were living on islands, lingering until about 4,000 years ago.

A definitive explanation for their mass extinction has yet to be agreed upon. The warming trend that occurred 12,000 years ago, accompanied by a glacial retreat, hunting,and rising sea levels, has been suggested as a contributing factor. Forests replaced open woodlands and grasslands across the continent. The available habitat may have been reduced for some megafaunal species, such as the mammoth. The spread of advanced human hunters through northern Eurasia and the Americas around the time of the extinctions was a new development, and thus might have contributed significantly.

The research used 1,323 mammoth radiocarbon dates, 658 peatland dates, 447 tree dates, and 576 dates from Paleolithic archaeological sites. Scientists from IoES and other UCLA departments obtained samples and worked on radiocarbon dating of the peatlands and the forests, and they created a database uniting information on hundreds of previously dated mammoth samples, developing the final map from thousands of dates and latitude and longitude records.

"It's a dramatic advance in the amount of data," said Wayne, who reconstructed mitochondrial DNA from radiocarbon-dated woolly mammoth remains. "Essentially, larger populations should have greater genetic diversity. However, in this case, the extent of fossil remains provided a more high-resolution picture of how the population size changed through time than genetic diversity."

Mapping the size and location of both mammoth and human populations alongside temperature changes and plant locations through time gave the researches a uniquely complete view of what happened, MacDonald said.

"We are, in a sense, time-traveling with our maps to look at the mammoths," he said.

What the new study was a vast map over geologic time showing the decline of the mammoths in different geographical areas.